Add default_ld_flags method

The -fobjc-arc flag should only be added when explicitly requested in 
the Podfile and the spec requires arc.

lib/cocoapods/generator/xcconfig.rb

@@ -40,6 +40,19 @@ module Pod
 
       private
 
+      # @return [String] the default linker flags. `-ObjC` is always included
+      #         while `-fobjc-arc` is included only if requested in the
+      #         Podfile.
+      #
+      def default_ld_flags
+        ld_flags = '-ObjC'
+        if target.target_definition.podfile.set_arc_compatibility_flag? and
+           target.spec_consumers.any? { |consumer| consumer.requires_arc? }
+          ld_flags << ' -fobjc-arc'
+        end
+        ld_flags
+      end
+
       # Converts an array of strings to a single string where the each string
       # is surrounded by double quotes and separated by a space. Used to
       # represent strings in a xcconfig file.

lib/cocoapods/generator/xcconfig/aggregate_xcconfig.rb

@@ -14,14 +14,9 @@ module Pod
       # @return [Xcodeproj::Config]
       #
       def generate
-        ld_flags = '-ObjC'
-        if target.target_definition.podfile.set_arc_compatibility_flag?
-          ld_flags << ' -fobjc-arc'
-        end
-
         config = {
           'ALWAYS_SEARCH_USER_PATHS'         => 'YES',
-          'OTHER_LDFLAGS'                    => ld_flags,
+          'OTHER_LDFLAGS'                    => default_ld_flags,
           'HEADER_SEARCH_PATHS'              => quote(sandbox.public_headers.search_paths),
           'PODS_ROOT'                        => target.relative_pods_root,
           'GCC_PREPROCESSOR_DEFINITIONS'     => '$(inherited) COCOAPODS=1',

lib/cocoapods/generator/xcconfig/private_pod_xcconfig.rb

@@ -13,14 +13,9 @@ module Pod
       # @return [Xcodeproj::Config]
       #
       def generate
-        ld_flags = '-ObjC'
-        if target.consumer.requires_arc?
-          ld_flags << ' -fobjc-arc'
-        end
-
         config = {
           'ALWAYS_SEARCH_USER_PATHS'     => 'YES',
-          'OTHER_LDFLAGS'                => ld_flags,
+          'OTHER_LDFLAGS'                => default_ld_flags,
           'PODS_ROOT'                    => '${SRCROOT}',
           'HEADER_SEARCH_PATHS'          => quote(target.build_headers.search_paths) + ' ' + quote(sandbox.public_headers.search_paths),
           'GCC_PREPROCESSOR_DEFINITIONS' => 'COCOAPODS=1',